4.4.3
RS-485 Termination Resistors
Termination resistors are required in long networks operating at the highest baud rates. Shorter
networks in high noise environments may also benefit from terminations. Networks as long 1000 ft.
operating at 9600 baud will function without termination resistors. Terminations should be
considered if the baud rate is higher and the network is longer.
When termination resistors are required, they are installed on the first and last station on the RS-485
wire pair. All other stations should not have termination resistors.
RS-485 networks are generally terminated with 120 ohm resistors on each end. The required 120
ohm resistor is built into the SCADAPack 32P controller. When these termination resistors are used,
the biasing generally has to be increased in order to generate at least 0.2V across the +/–Rx input.
To connect the termination resistors:
•
Install a jumper across J8 to terminate the Rx pair.
•
Install a jumper across J7 to terminate the Tx pair.
To disconnect the termination resistors:
•
Remove the jumper across J8 to terminate the Rx pair.
•
Remove the jumper across J7 to terminate the Tx pair.
4.5
RS-485 Wiring Examples
4.5.1
Four wire mode
Four-wire operation uses one pair of wires for transmitting data and a second pair for receiving data.
The transmitting pair is connected to +Tx and –Tx. The receiving pair is conRx and –Rx.
The following diagram shows the wiring details for a four-wire connection.
SCADAPack 32P Controller Hardware Manual
May 26, 2006
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